Zeolite exchangeable cation studies

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

The acidic character of 5A zeolite as a function of the calcium content has been explored by different techniques propylene adsorption experiments, ammonia thermodesorption followed by microgravimetry and FTIR spectroscopy. Propylene is chemisorbed and slowly transformed in carbonaceous compounds (coke) which remain trapped inside the zeolite pores. The coke quantities increase with the Ca2+ content. Olefin transformation results from an oligomerization catalytic process involving acidic adsorption sites. Ammonia thermodesorption studies as well as FTIR experiments have revealed the presence of acidic sites able to protonate NH3 molecules. This site number is also correlated to the Ca2+ ion content. As it has been observed for FAU zeolite exchanged with di- or trivalent metal cations, these sites are probably CaOH+ species whose vas(OH) mode have a spectral signature around 3567 cm"1. 

In general, the 2 1 clays are not very simple systems in which to study the interaction of water and surfaces. They have complex and variable compositions and their structures are poorly understood. Water occurs in several different environments zeolitic water in the interlayer regions, water adsorbed on the external surfaces of the crystallites, water coordinating the exchangeable cations, and, often, as pore water filling voids between the crystallites. Thus, there are many variables and the effects of each on the properties of water are difficult to separate. 

FAU type zeolites exchanged with many different cations (Na, K, Ba, Cu, Ni, Li, Rb, Sr, Cs, etc.) have been extensively studied. The unit cell contents of hydrated FAU type zeolite can be represented as M,j(H20)y [A Sii92 0384] -FAU, where x is the number of A1 atoms per unit cell and M is a monovalent cation (or one-half of a divalent cation, etc.). The number of A1 atoms per cell can vary from 96 to less than 4 (Si/Al ratios of 1 to more than 50). Zeolite X refers to zeolites with between 96 and 77 A1 atoms per cell (Si/Al ratios between 1 and 1.5) and Zeolite Y refers to zeolites with less than 76 A1 atoms per cell (Si/Al ratios higher than 1.5). 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

This table shows that it is diflicult, even in a model system, to present a simple view of the nature of the adsorption site because of the number of different parameters involved in the stabilization of OJ. For zeolites the problem is apparently more diflicult than for oxides, since not only do the framework ions and the exchanged cations form two distinct types of adsorption sites but the latter can migrate within the zeolite structure. It is difficult to obtain a full description of the coordination of the exchanged cations and so far there has been no systematic study on this point. 

Hi. Zeolites exchanged with transition metal ions. In the first row, scandium-, titanium-, cobalt-, and nickel-exchanged zeolites have been the most studied. Cobalt-exchanged zeolites are discussed in Section IV,E since they lead to oxygen adducts on adsorption of oxygen. There are several cases where copper and particularly iron ions are found as impurity cations which affect the oxygen adsorption properties of the zeolite. 

However, the influence of the exchangeable cation on the framework vibrations has not been systematically investigated. From x-ray diffraction studies (2) on zeolites it is known that most of the exchangeable cations are firmly bound onto the negatively charged framework. Therefore these cations might have some influence on the lattice vibrational modes. 

Cation Siting in Linde A. At the time this work was completed, x-ray studies on hydrated NaA (3, 4) and hydrated KA (5) had shown that 8 of the 12 exchangeable cations per unit cell are firmly bound to the zeolite framework and would therefore be expected to have the major influence on the lattice vibrations. These cations are sited in front of the sodalite... 

The properties of zeolitic water and the behavior of the exchangeable cations can be studied simultaneously by dielectric measurements (5, 6). In X-type zeolites Schirmer et al. (7) interpreted the dielectric relaxation as a jump of cations from sites II to III or from sites II to II. Jansen and Schoonheydt found only relaxations of cations on sites III in the dehydrated zeolites (8) as well as in the hydrated samples (9). Matron et al. (10) found three relaxations, a, (, and 7, in partially hydrated and hydrated NaX. They ascribed them respectively to cations on sites I and II, on sites III, and to water molecules. 

ST1 is a very favorable nucleus for solid-state NMR studies it has f = 2, high natural abundance and high sensitivity. Its large chemical shift range makes it possible to observe individual environments of the nucleus. Thallium can be easily introduced into zeolites by cation exchange. 

It is known that the 29Si NMR chemical shift in zeolites is sensitive to the type of the exchangeable cation (56), which indicates the presence of interactions between cations and the framework. In particular, the substitution of Na+ by Li+ in zeolite A and in synthetic faujasite moves the 29Si resonances ca. 4 ppm downfield in both cases. Melchior et al. (235) have used this effect to study the location of cations in a series of partially exchanged zeolites (Li,Na)-A. They found that the average 29Si chemical shift is not a... 

The extension of these studies to zeolites has yielded some interesting results. In an early study Egerton, Hardin and Sheppard (27) showed, in agreement with Ward s previous infrared results (28,29), that pyridine adsorbs mainly to the cation in a series of cation exchanged Y zeolites but that there is a linear shift of v] to higher frequency with the electrostatic potential (or charge to radius ratio, e/r, of the exchange cation). 

N diffuses into the structural pores of clinoptilolite 10 to 10 times faster than does CH4. Thus internal surfaces are kinetically selective for adsorption. Some clino samples are more effective at N2/CH4 separation than others and this property was correlated with the zeolite surface cation population. An incompletely exchanged clino containing doubly charged cations appears to be the most selective for N2. Using a computer-controlled pressure swing adsorption apparatus, several process variables were studied in multiple cycle experiments. These included feed composition and rates, and adsorber temperature, pressure and regeneration conditions. N2 diffusive flux reverses after about 60 seconds, but CH4 adsorption continues. This causes a decay in the observed N2/CH4 separation. Therefore, optimum process conditions include rapid adsorber pressurization and short adsorption/desorp-tion/regeneration cycles. 

The author of this book has been permanently active during his career in the held of materials science, studying diffusion, adsorption, ion exchange, cationic conduction, catalysis and permeation in metals, zeolites, silica, and perovskites. From his experience, the author considers that during the last years, a new held in materials science, that he calls the physical chemistry of materials, which emphasizes the study of materials for chemical, sustainable energy, and pollution abatement applications, has been developed. With regard to this development, the aim of this book is to teach the methods of syntheses and characterization of adsorbents, ion exchangers, cationic conductors, catalysts, and permeable porous and dense materials and their properties and applications. 

This rather unexpected result stimulated a number of other investigations, including the present one, with the conclusion that the 4 0 ordering scheme is correct.(13) In addition, the NMR and neutron diffraction data reported in referenced) have been re-interpreted in favor of 4 0 rather than 3 1 order,(14, 15) and it is concluded that the rhombohedral distrotion observed in the neutron pattern is strongly dependent upon the identity of the exchangeable cations. However, in an independent neutron study of Na-A zeolite(16), Adams and Haselden reported no evidence for such a distortion, and conclude that the symmetry must depend subtly upon the method of preparation. 

Silver species have been studied in a variety of A-zeolites including Nai2-A, K -A, Li -A, CsyNas-A and Cag-A (11). Complete exchange of cesium ion for sodium ion cannot be achieved in the A-zeolites. Typically the major cation was exchanged by silver to an extent of about 0.7 ion per unit cell which is 6% of the exchangeable cations. After irradiation about 0.003 silver ions per unit cell were converted to atomic silver species. 

Early studies, reviewed by Malquori (M81), showed that natural pozzolanas take up CH, including that produced by Portland cement, with the formation of products similar to those formed on hydration of the latter material. They also showed that the zeolites present in many of them were at least as reactive in this respect as the glassy constituents. Zeolites are cation exchangers, but the amounts of CaO they take up are much greater than can be thus explained moreover, cation exchange could not explain the develop-... 

A first stage of a systematic study of the natme of the low-pressure hysteresis loop observed in the nitrogen adsorption isotherms of some MFI zeolites is presented. It was shown that the pressure at which occurrence of this hysteresis loop takes place was linked to the presence of defect sites. The role of the exchangeable cation was also investigated, and it was suggested that in addition to the numbers of defect sites present, the strength of adsorbate-adsorbent interaction, and probably pore shape was also involved. 

A study was made of the ultraviolet spectra of benzene, alkyl-, amino-, and nitro-derivatives of benzene, diphenyl-amine, triphenylmethane, triphenylcarbinol, and anthra-quinone adsorbed on zeolites with alkali exchange cations, on Ca- and Cu-zeolites, and on decationized zeolites. The spectra of molecules adsorbed on zeolites totally cationized with alkali cations show only absorption bands caused by molecular adsorption. The spectra of aniline, pyridine, triphenylcarbinol, and anthraquinone adsorbed on decationized zeolite and Ca-zeolite are characterized by absorption of the corresponding compounds in the ionized state. The absorption bands of ionized benzene and cumene molecules appear only after uv-excitation of the adsorbed molecules. The concentration of carbonium ions produced during adsorption of triphenylcarbinol on Ca-zeolite and on the decationized zeolite depends on the degree of dehydroxyla-tion of the zeolite. 

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